In this study, we found that PBX1 was upregulated in tumor tissues of muscle-invasive BCa patients and BCa cell lines. Meanwhile, high expression level of PBX1 was positively related to tumor size, lymph node metastasis, distant metastasis and poorer survival status of muscle-invasive BCa patients. Besides, PBX1 mediated the growth, EMT progress, invasion ability and chemo-resistance of BCa cells. EMT is a necessary process in cancer metastasis, it promoted the BCa metastasis and recurrence [30]. Gugnoni et al. indicated that inducing the attenuation of EMT progress by CDH6 down-regulation dramatically depressed the metastatic behavior of papillary thyroid carcinomas [31]. Another study demonstrated that tumor-associated neutrophils secreted IL-17a and stimulated the EMT feature of gastric cancer cells leading to tumor malignant progression [32]. Consistent to our findings, it has been reported that PBX1 was involved in the EMT process of esophageal cancer and non-small cell lung adenocarcinoma via collaboration with ZEB2 and PREP1 [33, 34]. These results suggested that PBX1 may participated in BCa progression and may serve as a new prognostic and therapeutic target.
Previous studies revealed that PBX1 interacted with ER and promoted ER activation in breast cancer inducing the promoted metastatic progression in ERα-positive breast cancer patients [22, 23]. In our study, PBX1 showed no obvious effect on the expression of ERα and ERβ in T24 cells, but interacted with both ERα and ERβ. Deeply consulting the regulatory mechanism, we found that the inhibition of PBX1 suppressed the recruitment of ER to the enhancer of EGR3, cFOS and MYC in T24 cells. Evidence indicated that EGR3, cFOS and MYC were served as the target of ER transcription [35, 23]. In addition, cFOS and MYC were discovered to be oncogenes during BCa development, involving the regulations of cell proliferation, differentiation and invasion [36, 37]. These results indicated that PBX1 was required for the post-transcriptional regulation of ER activity in BCa cells.
It is convinced that BCa incidence was higher in man than in women, but female BCa patients exhibited advanced TNM stage, shorter survival time and poor prognosis [17, 38, 39]. Accumulated evidence indicated that estrogens promoted BCa tumorigenesis [40, 41]. The function of estrogens was mediated by its multiple receptors, including ERα and Erβ [38]. The agonists of ERα and ERβ, estradiol, enhanced BCa cell proliferation [20, 42]. Meanwhile, antiestrogens suppressed BCa cell growth, in vitro and in vivo [43, 44]. Consistent with previous studies, our study found that estrogen promoted cell proliferation and migration, while ER antagonist AZD9496 suppressed cell proliferation and migration, induced cell apoptosis in T24 cells. Besides, our study found that PBX1 aggravated estrogen-induced cell growth and invasion, while partially weakened the inhibitory effect of ER antagonist on cell growth and invasion of BCa cells. This may be due to the carcinogenesis of estrogens was depended on its receptors, while PBX1 was required for the ER activity in BCa cells.
CDDP is a largely used tumor chemotherapy drug in clinic that exerts anti-cancer activity by formatting inter- and intra-strand DNA adducts [45, 46]. Research data presented that 50% ~ 70% patients received therapeutic responses after CDDP-based treatment [47]. However, a tough issue occurred in the following therapy progression, in which tumor acquired a resistance to CDDP causing a high possibility of relapse [48]. Deeply efforts devoting on the potential mechanism of chemo-resistance might hold a great importance for solving the bottleneck of BCa therapy. Our research indicated that PBX1 was associated with the CDDP resistance of BCa patients and knockdown of PBX1 effectively promoted drug sensitivity of BCa cells via affecting ER signaling. Previously, PBX1 had been proved to be involved in the development of ovarian cancer chemo-resistance [49]. Takuro et al. discovered that tamoxifen modulated estrogen-mediated ER signaling pathway to enhance the drug sensitivity of BCa cells to CDDP [50], which greatly supported our findings.
PBX1 was upregulated in tumor tissues of muscle-invasive BCa patients and associated with tumor size, lymph node metastasis, distant metastasis and poorer survival status. PBX1 mediated BCa cell growth, EMT processes, invasion, cell apoptosis and CDDP resistance. In addition, PBX1 interacted with ERα and ERβ, and was required for ER function. Overexpression of PBX1 aggravated the tumor-promoting effect of E2 on T24 cells and partially suppressed the inhibitory effect of AZD9496. Hence, it is suggested that PBX1 participated in BCa progression and chemo-resistance through binding and activation estrogen receptors. This study provided a new therapeutic target for muscle-invasive BCa.